1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to an electrode design having a cathode active material of a relatively low energy density but of a relatively high rate capability and a second active material having a relatively high energy density but of a relatively low rate capability. The first and second active materials are short circuited to each other by contacting the opposite sides of a current collector. A preferred form of the cell has the electrode as a cathode connected to a terminal lead insulated from the casing serving as the negative terminal for the anode electrode. The present electrode design is useful for powering an implantable medical device requiring a high rate discharge application.
2. Prior Art
As is well known by those skilled in the art, an implantable cardiac defibrillator is a device that requires a power source for a generally medium rate, constant resistance load component provided by circuits performing such functions as, for example, the heart sensing and pacing functions. From time-to-time, the cardiac defibrillator may require a generally high rate, pulse discharge load component that occurs, for example, during charging of a capacitor in the defibrillator for the purpose of delivering an electrical shock to the heart to treat tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if left uncorrected.
It is generally recognized that for lithium cells, silver vanadium oxide (SVO) and, in particular, ε-phase silver vanadium oxide (AgV2O5.5), is preferred as the cathode active material. This active material has a theoretical volumetric capacity of 1.37 Ah/ml. By comparison, the theoretical volumetric capacity of CFx material (x=1.1) is 2.42 Ah/ml, which is 1.77 times that of ε-phase silver vanadium oxide. For powering a cardiac defibrillator, SVO is preferred because it can deliver high current pulses or high energy within a short period of time. Although CFx has higher volumetric capacity, it cannot be used in medical devices requiring a high rate discharge application due to its low to medium rate of discharge capability.
A novel electrode construction using both a high rate active material, such as SVO, and a high energy density material, such as CFx, is described in U.S. Pat. No. 6,551,747 to Gan. This application is assigned to the assignee of the present invention and incorporated herein by reference. FIG. 1 is a schematic view of a portion of a cathode electrode 10 according to the filed application. Electrode 10 is in an exaggerated, uncompressed condition and comprises spaced apart current collectors 12 and 14 supporting layers 16 and 18 of a first cathode active material on their respective outer major sides. The first cathode active materials 16, 18 are of a relatively high rate capability, but of a low energy density in comparison to a second cathode active material 20 sandwiched between and in contact with the current collectors 12, 14.
More particularly, the cathode active layer 16 has upper and lower sides 16A and 16B extending to and meeting with spaced apart left and right ends 16C and 16D. While not shown in the drawing, the sides 16A, 16B and ends 16C, 16D extend to and meet with a front side and a back side. Similarly, the cathode active layer 18 has lower and upper sides 18A and 18B and ends 18C and 18D extending to and meeting with a front side and a back side. For all intents and purposes, the layers 16 and 18 are of a similar shape.
The intermediate cathode active layer 20 has upper and lower sides 20A and 20B extending to spaced apart left and right ends 20C and 20D. The sides 20A, 20B and the ends 20C, 20D extend to and meet with a front side and a back side.
In an electrochemical cell (not shown), the first cathode active layers 16, 18 supported on the current collectors 12, 14, in turn, sandwiching the intermediate second cathode active layer 20 is compressed into a relatively thin assembly. In the compressed state, the ends 16C and 18C of the cathode layers 16, 18 extend beyond the ends 20C and 20D of the second active material layer 20. However, in the compressed state the ends 16C and 18 do not touch each other. While not shown, the front and back sides of the layers 16 and 18 also extend beyond the front and back sides of the intermediate layer 20, but in the compressed state they also do not touch each other. In the compressed state, the distal ends of the current collectors 12, 14 generally align with the left edge 20C of the intermediate layer 20.
With the cathode 10 shown in FIG. 1, it is possible for the cathode layers 16 and 18 to delaminate from the current collectors 12 and 14, especially in the vicinity of the ends 16C and 18C and the front and back sides. Essentially, potential sites of delamination exist wherever the layers 16, 18 extend beyond the peripheral edge of the intermediate layer 20 and of the current collectors 12, 14. The electrode construction of the present invention prevents such delamination from occurring.